1. Field
Aspects of embodiments of the present invention are directed to spin-transfer torque magnetoresistive random access memory devices.
2. Related Art
Magnetic memory, particularly magnetic random access memories (MRAMs), have drawn increasing interest due to their potential for high read/write speed, high endurance, non-volatility (e.g., persistence), and low power consumption. An MRAM can store information using magnetic materials as an information recording medium. One type of MRAM is a spin transfer torque random access memory (STT-RAM). STT-RAM uses magnetic junctions written at least in part by a current driven through the magnetic junction. A spin polarized current driven through the magnetic junction exerts a spin torque on the magnetic moments in the magnetic junction. As a result, layer(s) having magnetic moments that are responsive to the spin torque may be switched to a desired state. STT-RAM has the benefits of the fast read and write speed of SRAM, the capacity and cost benefits of DRAM, and the non-volatility of flash memory (e.g., persistence with zero standby power), coupled with high endurance (e.g., greater than 1015 cycles). As described below, STT-RAM uses a bi-directional current to write data. Such write operations may be performed without assistance from an externally applied magnetic field, heat, or other sources of energy. As such, STT-RAM has low energy requirements for writing.
FIG. 1 depicts a magnetic tunneling junction (MTJ) 10 as it may be used in a STT-RAM 50, as depicted in FIG. 2. The MTJ 10 may be disposed on a bottom contact 11 with a seed layer 12 and may include an antiferromagnetic (AFM) layer 14, a reference (or “pinned”) layer 16 having a magnetic moment 17, a tunneling barrier layer 18, a free layer 20, and a capping layer 22. FIG. 1 also shows a top contact 24. The top and bottom contacts 24 and 11 may be coupled to a selection device 62 (as shown in FIG. 2).
The STT-RAM 50 includes a magnetic memory cell 60 including the MTJ 10 and a selection device 62. The selection device 62 is generally a transistor such as an NMOS transistor and includes a drain 66, a source 64, and a gate 68. Also depicted are a word line 72, a bit line 74, and source line 70. The word line 72 is oriented perpendicular to the bit line 74. The source line 70 is typically either parallel or perpendicular to the bit line 74, depending on specific architecture used for the STT-RAM 50. The bit line 74 is connected to the MTJ 10, while the source line 70 is connected to the source 64 of the selection device 62. The word line 72 is connected to the gate 68.
The STT-RAM 50 programs the magnetic memory cell 60 by driving a bi-directional current through the cell 60. In particular, the MTJ 10 is configured to be changeable between high and low resistance states by a current flowing through the MTJ 10. For example, the MTJ 10 may be a magnetic tunneling junction (MTJ) or other magnetic structure that may be written using the spin transfer effect. This may be achieved by ensuring that the MTJ 10 has, for example, a sufficiently small cross-sectional area as well as other features desirable for switching using the spin transfer effect. When the current density is sufficiently large, the current carriers driven through the MTJ 10 may impart sufficient torque to change the state of the MTJ 10. When the write current, such as Iw1, is driven in one direction, the state may be changed from a low resistance state to a high resistance state. When the write current, such as Iw2, is passed through the MTJ 10 in the opposite direction, the state may be changed from a high resistance state to a low resistance state.
During write operations, the word line 72 is high and turns on the selection device 62. The write current flows either from the bit line 74 to the source line 70, or vice versa, depending upon the state to be written to the magnetic memory cell 60. The magnetic moment of the free layer 20 may thus be changed. During read operations, the column decoder (not shown) selects the desired bit lines 74. A row decoder also enables the appropriate word line(s) 72. Thus, the word line 72 is high, enabling the selection device 62. Consequently, a read current flows from the bit line 74 to the source line 70. In addition to the read current (IData in FIG. 2) flowing through the cell being read, reference currents are also driven through reference resistors. The output signals are provided to a sense amplifier. The value stored in the magnetic memory cell 60 is determined by comparing the current flowing through the cell being read to the reference current driven through the separate reference resistors.
However, using perpendicular SO for switching of perpendicular MTJ, as seen in FIG. 1, requires special surface alloy layers that can be more difficult to develop and demonstrate.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not constitute prior art.